JESSEN, B.; KARPF, J.; THORUP, A. Some Functional Equations in Groups and Rings. MATHEMATICA SCANDINAVICA, [S. l.], v. 22, p. 257–265, 1968. DOI: 10.7146/math.scand.a-10889. Disponível em: https://tidsskrift.dk/math/article/view/10889. Acesso em: 14 aug. 2026.